In UK waters, approximately every 12 hours 25 minutes. You may wonder why it is not exactly 12
hours, but you must remember that the Moon is also orbiting around the Earth. By the time a point on the Earth's surface has rotated from point x to point y (12 hours) the Moon has also moved a small amount, so the Earth has to rotate for an extra 25 minutes from point y to point z to be under the high water bulge.
